This project is aimed at elucidating the mechanism(s) and pattern(s) of fetal ovarian and testicular development in nonhuman primates. Particular attention will be paid to the development of the ovigerous and seminiferous cords, and to the origin and differentiation of the difinitive sustentacular and intersititial steroidogenic cells in the gonads of either sex. In male fetuses, the origin and the differentiation of the testicular excurrent pathways will also be studied. Two genera of primates will be included in the study: the rhesus macaque selected for the close morphological and functional similarities between its reproductive organs and those of the human, and the lesser bushbaby included in the investigation due to the fact that, in this as in other prosimians, oogenesis continues through adulthood. Gonads obtained together with their adjacent organs at close, sequential stages of development beginning with the gential ridge stage, will be prepared for electron microscopy. The gonads will be sectioned and studied by means of high resolution light microscopy (1Mu thick sections, stained with Toluidine blue) and electron microscopy. The study will concentrate especially on the role of the mesonephros in primate gonadal development: recent studies performed by us on ovarian and testicular morphogenesis in the sheep and the mouse have, in fact, demonstrated that in these species, the mesonephros is a key organizer of transitory and definitive gonadal structures and source of granulosa, Sertoli, Leydig and ovarian interstitial steroidogenic cells.